JPS5812328B2 - Fractional crystallization method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to improvements in aluminum refining,
More specifically, the present invention relates to improvements in fractional crystallization methods for aluminum purification.

Due to increasing awareness of the limits of natural resources, particularly energy resources, considerable efforts have been made to generate alternative sources of supply.

A source that appears to have particular promise for realizing this desire is the energy obtained from fusion reactors.

However, due to the need for isolation, or confinement, of the radioactive materials involved, considerable research has already begun to develop materials for reactors that do not present later disposal problems.

For example, if high-purity aluminum is used in the reactor, the radioactivity of such material will be reduced by a factor of 100 within a few weeks after shutdown, depending on the degree of purity of the aluminum.

In comparison, if stainless steel were used in the same application, this decay would take approximately 1,000 years, creating obvious difficulties in the disposal of such materials.

There is also growing interest in the use of high-purity and ultra-high-purity aluminum in stabilizing superconductors.

The cryogenic temperature at which the electrical resistance becomes very low in this application,
For example, it transports electrical energy at 4°K.

At such low temperatures, the higher the purity of the aluminum metal, the lower its resistance, ie the higher its conductivity.

The methods used in the prior art to purify aluminum are called preferential crystallization or fractional crystallization.

Such crystallization methods are described by Jarrett et al. in U.S. Pat. No. 3,211,547 and by Jacobs in U.S. Pat. No. 3,301,01.
This is disclosed in Specification No. 9.

These patents involve removing heat from the surface of molten aluminum to produce higher purity aluminum crystals in the impure molten aluminum.

Pure solid aluminum crystals are then tamped and packed into the bottom of the crystallizer.

The impure molten aluminum is then flowed out of the apparatus, followed by remelting the pure aluminum, and then varying the purity based on the degree of dilution by the impure molten aluminum contained between the crystals before remelting. It can be taken out in one piece or in several pieces.

In the accompanying drawings, Figure 1 schematically shows a cross-sectional elevation of a fractional crystallization furnace used in the method of the invention, and Figure 2 shows the impure fraction plotted against the percentage of the charge removed. It is a graph showing the concentration coefficient of silicon in aluminum.

According to the present invention, the surface of the impure aluminum body is heated to remove the eutectic impurities by (a) bringing the impure aluminum body into a molten state in a refining vessel; and (b) forming aluminum crystals. Once the crystals are removed, the crystals are of higher purity than the remaining liquid aluminum that makes up the part containing the concentrated impurities, and the crystals are removed from the surface that is removing the heat, and a portion of the crystals are placed in a container. Collect it on the floor near the bottom of the
and (C) heat is applied to the body near the bottom in order to melt some of the crystals collected near the bottom of the container, and the crystals are removed by the action of moving the crystals from the surface that removes the heat. The present invention provides an improved method for purifying impure aluminum by fractional crystallization, comprising the steps of moving the aluminum inside and the molten part transporting the impurities to the upper part of the body.

Referring now to FIG. 1, there is shown a vessel 60 for the improved fractional crystallization process of the present invention having an insulating wall 62 which can be heated if desired.

Preferably, the container has a layer 64 of powdered alumina that provides a barrier to molten aluminum that may escape through the inner wall 66.

Wall 66 should be made of a material that does not act as a source of contamination for molten aluminum 68.

The wall 66 is preferably made of high purity alumina, a refractory based on at least 90% by weight alumina, preferably 92 to 99% by weight.

Such refractories include Alundum VA
-112, Massachusetts (Massachusetts)
husetts), Worcester
This material, which may be purchased from Norton Company, Inc., is formed into the wall 66 in powder form and then sintered to provide rigidity.

This forms a unitary lining that is less likely to be penetrated by molten aluminum and is therefore more suitable for use in the bottom heating system of the present invention, as described below.

For example, when checking the material balance, 99
．． It showed a recovery rate of 7% by weight, indicating little or no penetration of the lining.

High purity alumina linings such as Alundum rarely cause contamination.

For example, the maximum contamination with iron or silicon is about 2pp Fe.
m and 3 ppm Si, some of which may be due to contamination of the outlet plug or the like.

Furthermore, sidewall solidification, which should also be avoided in order to produce high purity products, is less of a problem with such linings than in the prior art using materials such as silicon carbide and the like.

The temperature of the vessel walls is controlled by insulation or heating so that little or no heat flows out of the molten aluminum body.

Heat is extracted or removed at the free surface to solidify molten aluminum, which is called a solidification cycle.
This causes fractional crystallization of pure aluminum at the free surface of the molten metal and in the zone immediately below it.

If possible, solidification of the molten metal on the vessel walls should be prevented, or even if some solidification occurs, it should not exceed 10% of the melt.

In practice, molten aluminum solidifying on the vessel wall should not contaminate the fractional crystals occurring on the free surface or in the zone below.

In the method of the invention, molten aluminum is introduced into a vessel 60 for purification by fractional crystallization.

The aluminum source may be primary aluminum, typically consisting of 99.6J% aluminum, or a Hoopes cell.
It may also be a higher purity aluminum, such as 99.9% by weight aluminum, or even 99.993% by weight aluminum, as produced in an electrolytic cell known as 1).

U.S. Pat. No. 3,211 to Jarrett et al., supra.
As described in the '547 specification, in order to remove remaining impurities in the aluminum by fractional crystallization, high purity crystals of aluminum are produced in zone 70 as shown in FIG. Heat is removed from the molten aluminum at a rate that is maintained.

The pure crystals of aluminum thus produced were allowed to settle by gravity into zone 72, and after a predetermined amount of fractional crystallization had taken place, they were moved to the upper part of the vessel, leaving the remaining eutectic impurity-rich crystals. The impure molten aluminum 74 can be separated from the pure aluminum by flowing out through the upper outlet 76, ie, from the high purity aluminum.

The crystals are disintegrated by the action of a tamping device 78 which breaks up the bulk crystal product during the solidification process and also acts to compact the crystals in zone T2, as described in the Jarrett et al. Preferably, the settling process is accelerated.

After removing the impure molten aluminum mother liquor via the outlet T6, the vessel is heated to melt the pure aluminum crystals again, which are then removed via the lower outlet 80.

In accordance with a preferred aspect of the present invention, the crystals are packed or compacted during the solidification cycle to draw out impure liquid from between the crystals, generally in the bottom region 72 of the vessel.

Any impure liquid that has been displaced somewhat from the region 72 of the apparatus is removed via the upper outlet 76, thus forcing such liquid to flow through the lower region of high purity of the crystal bed, generally located at the bottom 72 of the apparatus. do.

It has been found that during the consolidation and consolidation cycle, a larger portion of higher purity aluminum can be obtained by heating the bottom of the apparatus during the consolidation cycle.

This heat can be supplied by an external induction coil or by a resistance wire or glober contained within a tube within the alundum lining.

A silicon carbide type glover, which can be purchased from Norton Corporation, mentioned above, can be used.

As pointed out long ago, the use of a unitary lining that prevents penetration of molten aluminum allows the use of such heating devices embedded in the lining.

For additional protection, each groper 110 can be inserted into a tube of a material that is not electrically conductive and impermeable to molten aluminum, such as mullite.

Although the heating device is shown at the bottom of layer 66 (FIG. 1),
It goes without saying that additional heating elements can be placed on the sides to obtain advantageous effects.

Heating at or near the bottom of the device during the solidification cycle, ie, while removing heat from or near the surface, can remelt some of the crystals near the bottom of the device.

This molten portion rises, ie moves through the crystal bed, carrying with it any remaining impure liquid.

Since the density of the crystal is greater than the density of the liquid phase or molten portion, the molten portion rises or moves upward through the crystal, with the crystals tending to displace the molten portion at or near the bottom of the device. It is thought that this will be promoted.

Moreover, bottom heating during the packing, i.e., consolidation process,
It is very advantageous that the molten part can be pushed up through the crystal bed, entraining any impurities that remain between the crystals or adhere to them! The bottom heating prevents solidification of the bottom liquid phase containing impurities that may adversely affect the purity level when the entire crystal is finally melted again for removal through the lower outlet 80. It is also advantageous in that it can be done.

It goes without saying that bottom heating should normally be carefully controlled during the hardening cycle to prevent excessive remelting.

Typically, heating at or near the bottom during the solidification cycle is limited to a heating area of 1 square centimeter, depending on the removal of heat in the vicinity of the surface and the insulation value of the walls, to some extent. Hit i. iW (ik per i square foot
The heat should be controlled to be introduced at a rate not substantially less than W).

Typical heating range at the bottom of the device is 0.54-3.2W
/sq. senna (0.5-3.0 kW/sq. ft.).

It is noted that the bottom heating rate is usually limited to a fraction of the rate of heat removal.

Typically, we have found that best results are obtained when the remelting rate at or near the bottom of the apparatus is controlled to be within about 5 to 25% of the crystallization or solidification rate.

However, these speeds may be lower or lower depending on the pressure used for packing and the density of the crystal bed.

The advantage of using controlled heating near the bottom of the vessel for controlled remelting of the crystals is that the impurity levels for silicon that can be obtained with or without bottom heating are Clearly explained with reference to figures.

That is, FIG. 2 shows the silicon concentration coefficient (the ratio of the impurity concentration in the sample to the impurity concentration in the raw material) plotted against the amount of aluminum taken out from the crystallizer.

For example, if the initial concentration of silicon in the device is 360 ppm and its concentration coefficient (cp) is 1, then from Figure 2 it can be seen that the concentration of silicon for the amount of aluminum removed by using bottom heating is , compared to the concentration of silicon using a normal solidification cycle (3.7)
It is pointed out that

A high concentration factor is important, first of all, in that a greater amount of impurities can be removed via the upper outlet, as can be seen in FIG.

Second, a very small amount of gold must be removed to significantly reduce the impurity level (approximately 30% in the example shown in Figure 2).

Thus, it can be seen from FIG. 2 that in a typical solidification cycle, approximately 60 to 70% of the charge must be removed to remove the same amount of impurities.

However, in the present invention, about 60% of the raw material can be obtained as a high-purity product.

It can be seen that by using bottom heating the yield of refined metal can be significantly increased.

Referring to FIG. 2 as an example, it is pointed out that the yield can be doubled.

It goes without saying that higher concentration factors can be obtained by varying the packing pressure and bottom heating.

That is, the impurities can be further controlled and even higher yields can be obtained by removing a smaller portion via the upper outlet.

It is not clear why bottom heating and compaction exhibit any advantages in terms of yield, but if carried out in this way, the purity coefficient for iron, for example, can be theoretically explained using a binary phase diagram. It was pointed out that the results are much higher than what can be achieved.

For example, Baya, which has an iron content of 0.05% by weight,
In the binary phase diagram, the purest substance is FeO. which corresponds to the highest purification factor of 37. It should contain 0.0014% by weight.

However, when experiments were performed using the method described above, some materials contained less than 0.0005% by weight Fe and FeO. There were even values as low as 0003% by weight.

It appears that this particular purification can only be explained by the replacement of the original liquid by a purer liquid due to the mechanism of bottom heating and packing.

The crystal then equilibrates with the purer liquid according to the theoretical distribution relationship.

In other words, it is thought that a mass transfer phenomenon occurs in the solid state through the solid crystal and from the solid crystal to the high-purity liquid phase surrounding the crystal as it reaches equilibrium with the liquid phase.

The solidification, or crystallization cycle, can be allowed to occur over a period of about 2 to 7 hours.

Heating of the bottom of the apparatus can be extended for the same amount of time to partially remelt some of the crystals near the bottom of the bed 72 (FIG. 1).

However, bottom heating is part of the solidification cycle).
We have found that this can typically only be done during about the last two-thirds of the solidification cycle.

It has been found that as well as using bottom heating during the solidification cycle, such heating is advantageous during remelting of the crystals for harvesting them from the fractional crystallizer.

That is, in addition to remelting the very pure product crystals by normal surface heating, heat is applied to the bottom of the device in the same manner as described above.

The use of bottom heating during the remelting cycle has the advantage that it prevents solidification at or near the bottom of the vessel where the liquid phase in high purity products can interfere with purity levels.

Furthermore, having the high purity product in molten form facilitates the opening of the lower outlet.

Additionally, bottom heating reduces the time required to melt the crystal bed in the device, significantly increasing the overall economy of the system.

Typically, about 2 to 5 hours are required to melt the crystal bed.

The following examples further illustrate the invention.

EXAMPLE Approximately 2000 pounds of aluminum alloy containing 360 ppm silicon and other impurities was charged to a crystallizer substantially as shown in FIG.

To simplify the illustration, only silicon will be traced.

The charge is first melted and then approximately 7.5 W/sq.
kW/sq ft), but was removed by blowing air over the surface.

After about one hour of operation, the bottom heating element was activated to apply heat to the bottom of the vessel at a rate of about 1.1 W/senna (about 1.0 kW/sq. ft.).

The blade of the tamping device was depressed into the device at approximately 2 second intervals to prevent crystal bed formation at or near the surface of the melt.

After sufficient crystal formation, packing the crystals into the lower part of the apparatus and moving the liquid phase toward the upper part of the apparatus;
At the same time, the blade was pressed down (approximately every 2 seconds) to carry away impurities.

The blade pressure ranged from 0 to 20 pounds per square inch and increased as the crystal bed formed.

It is noted that bottom heating melts crystals at or near the bottom of the vessel to produce high purity aluminum and expels the crystals as the high purity aluminum moves upwardly into the upper region of the vessel.

Remove from heat and cook for about 3 hours, and about 70% of the prepared product.
After crystallization, the first metal removed from the opening of the brush was analyzed for silicon concentration.

This sample corresponds to the zero preparation product taken out in FIG.

Samples of the charge were then taken substantially as shown in FIG.

That is, by examining FIG. 2, it can be seen that approximately 33% of the charged product was taken out through the upper takeout port.

Furthermore, by examining the curves, it can be seen that using bottom heating during the solidification cycle concentrated the silicon much more than using conventional methods, especially in the part of the curve relating to the upper outlet.

It is noted that the greater the amount of impurity that can be concentrated for removal via the top outlet, the lower the amount present when removing metal via the bottom outlet.

In this way, a large amount of silicon impurities was extracted through the upper outlet compared to the normal method, so the metal component extracted through the lower outlet was extracted through the lower outlet in the usual way. You can tell that the purity was much better than that of the original.

From FIG. 1, it can be seen that in general, the yield according to the present invention is about twice that of conventional methods.

Although only silicon is used to illustrate the invention, it will be appreciated that the effects shown in Figure 2 are the same for any other eutectic impurity that may be encountered.

Additionally, the conventional solidification cycle curve referenced in FIG. 2 was obtained in the same manner as described above, except that no bottom heating was used.

Moreover, FIG. 2 illustrates that higher yields can be obtained by concentrating impurities into a smaller volume of metal.

[Brief explanation of drawings]

Figure 1 is a schematic cross-sectional elevational view of the fractional crystallization furnace used in the method of the present invention, and Figure 2 shows the percentage of the charged material taken out and the concentration coefficient of silicon contained in impure aluminum. 60 is a container, 62 is a heat insulating wall,
64 is an alumina powder layer, 66 is an inner wall, 68 is molten aluminum, 70 is a crystal with a high aluminum content, 72 is a bottom region of the container, 74 is impure molten aluminum, 76 is an upper outlet, 78 is a tapping device, 80 is a lower outlet, 100 is a mullite tube, and 110 is a glover.

Claims (1)

[Claims] 1 (a) Bringing the impure aluminum body into a molten state in a refining container; (b) Producing the surface of the impure aluminum body in order to remove eutectic impurities by forming aluminum crystals; The heat is removed, the crystals are of higher purity than the remaining liquid aluminum that constitutes the part containing the concentrated impurities, and the crystals are moved away from the surface from which the heat is removed and the part of the crystal is purified. (c) applying heat to the body near the bottom to melt a portion of the crystals collected near the bottom of the container; A method for refining impure aluminum by fractional crystallization, characterized by a process in which the crystal is moved in the crystal by an action that moves the crystal away from the removed surface, and the molten part carries the impurity to the upper part of the body. 2. Process according to item 1, characterized in that the crystals are packed by kumping in order to bring out the remelted part and the remaining part containing concentrated impurities from the crystal bed to the upper part of the body. 3. The method according to item 1, wherein the liquid aluminum containing concentrated impurities is removed from the container through an upper outlet of the container to prevent contamination of the high-purity crystals. 4. After removing the impure portion, the aluminum crystal is melted again, and the remelting is promoted by applying heat near the bottom of the crystal bed.
The method according to any one of Items 1 to 3. 5 In step (c), 0.54 per square centimeter of heated area
3. The method according to any one of paragraphs 1 to 3, characterized in that heat is applied at a rate of ~3.2 W. 6. In step (c), substantially 1. 4. A method according to any one of the preceding paragraphs 1 to 3, characterized in that heat is applied at a lower rate than IW. 7. The method according to item 1, wherein in step (C), heat is applied at a rate of 5 to 25 inches of the rate at which heat is removed in step (b). 8. The method according to item 1 above, wherein heat is applied in step (c) after starting the removal of heat in step (b).